A major problem of industry, and the electric power companies in particular, is that their processes require cooling water in great quantity. This has preferably been done in the past by locating plants beside major rivers and pumping out some of the river water and, after the water has been used and heated, returning it to the river to flow down stream. Because of the great expansion of industry, and especially expansion of the power companies, many rivers have been heated to the point where fish have been killed and the temperature of the river water has been raised to a temperature where it is unsatisfactory to the subsequent plants down stream that use the river for cooling.